DE2240673A1 - PROCESS FOR MANUFACTURING POROUS MEMBRANES OR COMPOSITE SEPARATING LAYERS FOR GAS DIFFUSION SYSTEMS - Google Patents

Description

Dit. i \ (j. II. NE (IENI) ANK · «ipTj.-ixg. H. HAITCK · i> ipl.-phys. W. SCHMITZ dipl-ing. E. ORAALFS · dipping. W. WBENBRT 2240673

HAMBURG-MUNICH XTJSTlOI. I, C XGS ANSCIIItIFT: ΙΓΑΜΒΤΤΚΟ 3β

TEL. 30 7 1 28 AND 30 11 IS

TELEGH. NEGEUAPATKNT HAMBUItU

. '> ■ - .. ·'',f' / YIT.ü 'EDISOIi .jpA münchex in ■ mozartsth. 33

i'oro i-jiionaparte i-.ai.l and / Italy-

TEL. P. 38 05 80

IiOt ¹ O i'j'ionaparxe _) \ , tblegh. jjegedapatent Munich

. August 14, 1972

Process for the production of porous membranes or composite separating layers for gas fusion systems

The present invention relates to a method of delivery of composite separating layers (or porous membranes), which are separated by one on a support, which is in the is designated as macroporous, fixed microporous layer are formed.

In particular, the present invention relates to a Method of depositing a sub-electronic (i.e. with an average diameter below 7 λθ aluminum powder obtained microporous layer on the inner walls of support tubes made of porous aluminum oxide nind. . .

The separating layers suitable for use in gas diffusion systems or porous membrane must have the following general properties, among others: Extremely reduced Pore size and high permeability.

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Thus the need arises to determine the thickness of the fusion membrane, which is referred to below as microporous, as much as possible to reduce what is achieved a manufacturing productG with satisfactory mechanical properties. The properties mentioned can be obtained by placing said micropxous layer on a support of greater thickness but substantial forms larger pore dimensions. Separating layers of this type are called composite separating layers.

Some types of aluminum oxide, e.g. in the form of gels, are already used in the manufacture of (either tubular or flat) separators have been used for gas diffusion applications in general and especially for the enrichment of uranium isotopes

Microporous layers have also been obtained by anodic oxidation of aluminum metal sheets or by prepressing of fine granules (with grain sizes from one hundredth to a few hundredths of a micron) of aluminum oxide, titanium oxide or other similar materials against certain substrates such as metal nets etc.

The main object of this invention is therefore to provide composite separating layers made entirely of sintered porous alumina for gas diffusion based applications.

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In particular, "the invention aims to provide a Process for making said composite release liners having uniform microporous adhered to the support Layers that have an adjustable thickness according to the working conditions and, above all, free are from defects such as cracks, cracks or peeling of the layer from the carrier.

These and other goals set out in the course of the description below more clearly are achieved in a convenient manner by a method according to the invention comprises the following steps: producing a carrier, preferably of tubular shape, made of sintered porous aluminum oxide, application of a microporous layer the carrier by adding an aqueous suspension that is submicro-

Niche particles of aluminum oxide and / or aluminum hydrates, preferably in concentrations of about 0.1 to about 10 %, as well as at least one methyl cellulose (with different degrees of methylation) or other hydrophilic ethers of cellulose, preferably in concentrations between about 0.1 % and about 2 %, filtered through the porous walls of the support, this filtering being carried out from the inside to the outside of the preferably tubular porous support and under a suitable pressure, removing excess suspension from the inside of the support, allowing the layer to dry, calcining the resultant Product in air up to the removal of the

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combustible substances, and finally sintering the product at temperatures above about 90O ⁰ C, but preferably between about 95O ⁰ C and about 1250 ⁰ C, where optionally the microporous layer after calcining and before the final sintering, a compression in the cold by means of a is subjected to isostatic pressing of the manufactured body.

In particular, the method according to the invention provides for the use of a stabilized and degassed suspension and may require - around the settling of alumina powder to avoid inside the larger pores of the support - that the inner surface of the support before mentioned Filtration process by dry rubbing with very fine aluminum oxide powder or other correspondingly easy removable materials such as carbon black or the like can be treated appropriately.

As previously stated, this invention relates to the preparation of composite alumina release liners, preferably of the tubular type, i.e. that these composite layers consist of a porous alumina support and one on the inner Surface of the support fixed layer consist of microporous aluminum oxide. The method according to the invention allows however, with readily recognizable modifications or changes, the layer on the outer surface of the Form carriers or planar or otherwise geometrical manufacture molded composite release liners.

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One type of alumina powder which has shown particularly good results in accordance with this invention is submicron; Powder of gamma-alumina known under the trade name ⁿ Alon "and manufactured by Cabot Co., USA ·

This powder is very easy to disperse in water and the suspension thus obtained is sufficient stable. The use of known dispersion techniques, e.g. by means of ultrasound, can be useful, but is not indispensable.

The application of the microporous layer inside the support tubes is in practice appropriate by means of filtration Amount of a suspension of the type described above with the use of suitable ones to be described later Aids run through the porous walls of the support.

Indeed, it was observed that the precipitate obtainable with the aid of a simple aqueous suspension of ¹¹ AlOn ¹¹ suffered from noticeable cracking during drying, and this disadvantage was also found in the case of aqueous suspensions of other powders which, due to their composition and degree of delicacy, were more or less are less suitable and made of different types of aluminum oxide

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and alumina hydrates. Also the use of anhydrous liquid media does not lead to better results, which is due to the lower tendency to form stable suspensions and due to the fact that the precipitate obtained turned out to be soft and little turned out to be adherent.

On the other hand, the addition of various additives and binders as well as the adjustment of the pH value also has a role Acids or bases did not bring about any significant improvements.

Conversely, it has been found that the presence of hydrophilic ethers of cellulose, such as methylsilulose and the like, in concentrations between about 0.1 and about 2 %, in an aqueous suspension containing submicron particles of alumina and / or alumina hydrate, and preferably ^N Alon ^N in concentrations between about 0.1 and about 5%, have been found to be particularly effective in inhibiting the formation of cracks during the drying of the layer produced by filtering.

According to the invention, the precipitate is by filtration of said suspension from the inside to the outside of the The carrier tube is brought about by means of a suitable pressure difference (i.e. between 0.05 and 1 atmosphere).

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It is advisable to keep the tube filled with the liquid during the entire filtration process. It is alike advisable that the suspension used has been previously used in a suitable manner, e.g. in a vacuum, is degassed. It's very easy, to obtain the desired thickness of the deposited layer by determining in advance the amount of the Liquid based on the concentration of alumina in the suspension, the internal surface area of the support and the apparent (final) density of the material, which is to form the microporous layer, determines »The amount of Liquid filtered through the walls of the carrier is more conveniently located upstream of the carrier (e.g., through a volumetric pump, by level drop or by other suitable systems) instead of directly from the filtered one Liquid determined, since it is expedient to continuously add water to the outer surface of the carrier spray. At the end of the filtration, the one in the tube becomes Drained off the liquid, and the still moist precipitate remains.

In order to avoid the potential disadvantage that the powder also percolated through the relatively large pores of the support, the inner surface of the carrier may suitably before the filtration process by rubbing with dry ^w Alon "powder

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treated to clog the entrance of the pores; the excess powder is then passed through a stream of air removed. Alternatively, the pore entrances can also be clogged suitably by using very fine instead of "Alon" Powder made from substances that can be removed by incineration in air used, such as carbon black or other similar substances.

After drying, the SU is not uniform, adhesive, shiny and free from cracks provided the layer thickness is limited to no more than a few tens of microns.

The tube with the deposited layer is then calcined in air (at between about 500 and about 800 ⁰ C), so that the residual Methylailulose and the other possibly added substances are removed.

At this point the filtration and subsequent calcining process be repeated if one wishes to obtain layers with greater thicknesses: In this way, the layer obtained in several stages as still uniform and free from the defects that occur in the case of direct precipitation a thick layer can be observed (cracks, peeling, etc.).

After sintering in air between about 950 and about 1150 ° C the layer thus obtained shows a structure with predominantly open porosity and pores with an average

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Radius tain etv / a 150 8 (measured with a mercury porosity meter). However, Ijb is still a considerable proportion (etv / a 20% by volume) of significantly larger pores. ·

Conversely, the size distribution of the pores will be within a narrower range and focus around lower average values obtained, if after calcining and before the above-mentioned final sintering treatment, the layer a cold compression (above 700 kp / cm) is subjected. For this purpose it is convenient to have the product an isostatic To subject the pressing process, which can be carried out by placing the tube in an apparatus as it is described as an example and not limiting for the invention in the accompanying drawing.

The apparatus shown schematically in the single figure of the drawing contains two sealing plugs 5 at the ends of the porous aluminum oxide support tube 1, on the inner surface of which the microporous layer 2 was deposited according to the technique described above; this microporous layer 2 must be pressed against the support tube 1. Furthermore, the apparatus contains a deformable tubular membrane 3 within the aluminum oxide tube (1 and 2), a likewise deformable outer tubular membrane 4, two rigid fitted annular bearings 6 with an inlet 9 for the pressure fluid 8 and two outer, drawn-on retaining rings 7.

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The whole apparatus is brought into the interior of an accessory not shown in the drawing, see above that the pressure of the pressure fluid 8 uniformly on the entire apparatus (both outside and inside) to act can be brought. In this way, the deformable membranes transfer the outer and inner surfaces of the aluminum oxide tube (1 + 2) and thereby isolate the hydraulic fluid, the pressure in such a way that that the deposited layer 2 is pressed against the inner surface of the support tube 1. From this is deduce that the structure of the support tube also

and

is subject to compression / that its resilience sets a limit to the maximum applicable pressure. It is not necessary to remove the air that remains trapped in the spaces between the membranes 3 and 4, to vent if the decrease in volume during compression does not result in an excessive increase in pressure Consideration of the volume available within the highly porous carrier material.

There are no particular limits on the type of material that constitutes the outer membrane 4 (e.g. elastic rubber search), and contact with the If necessary, the support tube can be lighted by interposing it of neutral or removable material (powder or paper sheets or other material) should be avoided. Conversely, the choice of material that is applied to the

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Deposited layer 2 represents working inner tubular membrane / is quite critical.

Many materials (vulcanized natural rubber, silicone « rubber, para rubber, etc.) have a strong tendency to stick to the Layer 2 and cause their detachment or change in the depressurization phase or during dismantling. Uncured natural rubber obtained from pure latex turned out to be one of the most suitable Materials. Materials such as Cellphan or polyethylene show a limited gluing, but are more difficult to use directly than dense deformable tubular membranes. Thin sheets can be advantageous These materials are used to isolate the inner tubular membrane from the layer because they are relatively are easily detachable or easily removable by calcining.

However, it is more convenient to have a removable material in powder form (e.g. soot, etc.) to be inserted. At the end of the compression process, this material appears against the aluminum oxide layer condensed and is then used during the following ^. Treatment phase by burning in air at high temperature removed. On the other hand, it is recommended that the amount of powder used for this insulation be as low as possible to measure so as not to uselessly change the purity of the layer, among other things. E-; has shown in practice

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2 2 4 over 6 7 3

that it is sufficient to cover the surface i e.g. " by simply rubbing dry in order to prevent the membrane from sticking to the deposited layer.

The pores that were obtained after compaction at about 460 kg / cm and after the subsequent sintering in air at about 1050 ^° C. (for 3 hours) were predominantly monodisperse and had a mean diameter of less than about &. Similarly, compaction at about 1000 kgf / cm results in a mean diameter of less than about 200 &. Compressions at higher pressures cause a further reduction in the mean pore diameter (for example about 120 8 at about 4000 kg / cm ² and sintering at about 1000 ^° C.).

Example No. 1:

An aqueous suspension was first prepared which contained 20 mg / cnr of the very fine aluminum oxide type "Alon" from Cabot Corp., USA, with a specific surface area of 100 m ² / g.

Separately, a clear aqueous solution of Ilethocel MC 4000 from DOW CHEMICAL Co. USA was prepared at a concentration of 5 mg / cnr. Methocel MC 4000 is a dimethyl ether of cellulose, containing from 27.5 to 3 ^, 5% methoxy groups, and with a viscosity of approx. 4000 cP (measured in a

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2% ± gcn solution at 20 ⁰ C). For this purpose, an equal volume of the "Alon" suspension described above was added with constant stirring, resulting in a homogeneous suspension with a content of approx. 10 mg / cm- ^5, corresponding to approx. 1 % "Alon" and 2.5 mg / cm ³ Methocel MC 4000.

The suspension was then degassed at room temperature in a container in which gradually reduced to vacuum was generated with a residual pressure of approx. 40 mm Hg.

A support tube with an outside diameter of 1.8 cm, a thickness of 0.15 cm and a length of 37 cm porous alumina with an open porosity of 46% and an average pore diameter of approximately 3 microns was soaked in water by filling it and then emptying it.

The support tube was then completely filled with the suspension described above in a vertical position from below filled. As soon as all of the air had escaped from the upper opening of the tube, this opening was closed and the tube was flipped to a horizontal position while the suspension was continuous at the other end of the Tube was introduced with a static pressure of 1.5 m water column through a narrow feed pipe. After this

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40 cra ^ ¹ suspension had been filtered, the tube was returned to the vertical position, the introduction of the suspension was interrupted, the upper end of the tube was opened and the suspension contained in the tube was emptied through the lower end of the tube.

After drying at room temperature, the tube was gradually heated in a muffle furnace for 4 hours at 65O ⁰ C and kept at this temperature for 45 minutes. After a slow cooling in the muffle furnace, the pipe was assembled as described above for the compaction of the layer against the inside of the loader. This compaction was carried out by applying a pressure increasing to 1000 kg / cm. Because the pressure was relieved and the pipe was released from the structure. It was again ^{heated to 1050 °} C. in air and left at this temperature for 3 hours, after which it was allowed to cool in the oven.

The layer produced in this way shows an average layer thickness of approx. 20 microns and was due to an open porosity of approx. 65% with a narrow pore size distribution and with a mean pore diameter of less than 200 8 ocharacterized.

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Example No. 2

A disc of sintered aluminum oxide (0 = 2.6 cm, thickness 0.5 cm) with pores of a mean diameter of 3 microns and an open porosity of k5% was placed on a surface with a cotton swab and some ¹¹ Alon "- After the excess powder was removed with an air jet, the amount of that contained in the cavities of the disk surface was determined

2
Powder about 1 mg per cm of surface. The disc was then placed in an annular rubber container so as to ensure a tight seal with the side disc surface, and it was then carefully soaked in distilled water. On its upper surface, after treatment with "Alon" powder, 2 cm- 'of an aqueous Alon suspension which had a concentration of 2.5 mg / cm ^ "Alon" and contained 2.5 mg / cm * Methocel MC 4000 , evenly distributed, the suspension having been prepared according to the method described in Example 1. At that moment, a vacuum of 400 mm Hg was applied to the underside of the disk, causing the liquid to be filtered. The disk was then removed from the ring-shaped container, air-dried, then slowly ^{heated to 700 °} C. in air and finally slowly cooled.

The layer obtained in this way was then applied by means of mechanical compression at a pressure of 460 kg / cm

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the disk plane, pressed against the carrier, with between the disc and the lower and upper metal plates of the press a 5 mm thick layer of para rubber was placed, which in turn was isolated from the pane by cellophane systems with a layer thickness of 4 / 100mm.

The disk was then treated in the same way as described in Example 1 at 1050 ^° C. for 3 hours in air. The layer obtained in this way had a thickness of approx. 10 microns and had an open porosity of approx. 70-6 a mean pore diameter of $ 230.

Example No. 3

According to the method described in Example 1, a suspension containing 2.5 mg / cnr "Alon ¹¹ and 1 mg / cnr Methocel MC 4000 manufactured. According to the same procedures as in the above example, 80 ⁵ cm- suspension from the inside to The outside of an identical support tube was taken for filtration and the tube was then treated further as in the example above The resulting layer showed properties similar to those described in Example 1 and had an average thickness of about 10 microns.

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Example No. 4;

The procedure was as in Example 1, but a suspension was used which contained 10 mg / car "Alon" and 10 mg / cm ^ Hethocel HC 25 from D0 ¥ CHEMICAL Co. Methocel MC 25 is a dimethyl ether of cellulose with a viscosity of 20-30 cps in a 2 $ cent, aqueous solution at 20 ⁰ C. The thus erhaltene.Schicht showed CHARACTERISTICS similar to those described in Example. 1

Example no.5:

The procedure was as in Example 1, but a suspension was used which contained 7.5 mg / cm- ⁵ "Alon" and 5 mg / cm-5 Hethocel 65 HG 4000 from DOV / CHEMICAL CO. contained. This Hethocel is a cellulose ethyl ether in which the methoxy groups (-OCH,) are partially replaced by propylene glycol groups (-OCH ₂ .CHOH.CH ₃ ). It contains 27-29 ^ methoxy and 4-7.5 $ propylene glycol ether groups.The viscosity of Methocel was between 3 x 500 and 5,600 cps in a?: 2 4igen aqueous solution at 20 ⁰ C. The layer thus obtained exhibited properties similar to those described in Example 1, with an average layer thickness of about 15 microns.

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Example No. 6;

The procedure of Example 1 was repeated with the exception that a up to kg / cm increasing pressure was applied in compressing the layer against the inner wall of the support tube 2000 and Con t de temperature of the final heat treatment 1000 ⁰ C.

The microporous layer obtained in this case has an average thickness around 18 microns and an open porosity around βθ% with a narrow pore size distribution and an average pore diameter of less than 170 8.

Example no.7:

The procedure according to Example 1 was repeated with the exception that the carrier tube was made of two layers of porous sintered sintered layers The outer layer has a density of 0.14 cm and an open porosity by 40% and an average pore diameter is characterized by 7 microns, while the inner layer has a thickness of 0.01 cm and an average pore diameter around 2 microns. The microporous layer obtained in this case has the same characteristics as in example 1.

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Obviously, the invention which has been described and illustrated can be modified in practice and variants of equivalent value introduced. Equally, the construction of the compacting apparatus can also be varied by taking into account the type and shape of the supports and to be treated, without these modifications and variants being outside the scope of the invention.

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Claims (8)

Patent claims: 1. Method of making composite porous Separation layers or membranes made entirely of sintered porous aluminum oxide for applications on the Area of gas diffusion, characterized by the following steps: Making a carrier, preferably of tubular shape, from sintered porous aluminum oxide; Applying a microporous layer to the support by adding an aqueous suspension containing submicra particles of aluminum oxide and / or alurainium hydrates, preferably in concentrations of about 0.1 to about 10? &, And at least one hydrophilic cellulose ether, preferably in concentrations of about 0 , Contains 1 to about 2 Jo, filtered through the porous walls of the support, this filtering being carried out from the inside to the outside of the preferably tubular support under a suitable pressure; Removing the excess suspension from the inside of the support and allowing the layer to dry; Calcining the produced body in air until the combustible components are removed; and schlMJlich sintering the product in the air at temperatures above about 900 ° C, preferably but optionally, the microporous layer is subjected to after KaMnieren and before the final sintering of a cold compression between about 950 ° and about 125O ⁰ C. 309811/099 * -21- 2. The method according to claim 1, characterized in that the aqueous suspension of methyl cellulose with different Contains degrees of methylation. 3. The method according to claim 1 or 2, characterized in that that the inner (inside) surface of the carrier before the filtration process preferably by dry rubbing with alumina powder and / or powders from others removable materials such as carbon black. 4. The method according to any one of the preceding claims, characterized in that the filtration processes and the subsequent Calcining can be repeated to obtain microporous layers of greater thickness. 5. Method according to one of the preceding claims, characterized in that that the compression process in the cold thereby it is carried out that the body produced is an isostatic Compression inside a suitable apparatus is subjected, with the body produced between two deformable tightly fitting membranes are arranged, of which the one pressing against the microporous layer is a membrane preferably made of pure, unvulcanized and pure Latex obtained natural rubber is made. - 22 - 309811/0998 6. The method according to any one of the preceding claims, characterized in that the compression by pressing more than about AOO kg / cn. 7. The method according to any one of the preceding claims, characterized in that the calcination is carried out at temperatures between about 500 ° and about 800 ⁰ C. 8 .. Composite porous separating layers and membranes
sintered aluminum oxide, characterized in that they are obtained either in tubular or flat shapes by the method described and claimed. 309811/0998